Hydrophobicity and molecular weight
(MW) are two fundamental properties
of dissolved organic matter (DOM) in wastewater treatment systems.
This study proposes fluorescence Stokes shift and specific fluorescence
intensity (SFI) as novel indicators of hydrophobicity and MW. These
indicators originate from the energy gap and photon efficiency of
the fluorescence process and can be readily extracted from a fluorescence
excitation–emission matrix (EEM). The statistical linkages
between these indicators and hydrophobicity/MW were explored through
investigation of DOM across 10 full-scale membrane bioreactors treating
municipal wastewater. Stokes shift was found to exhibit a general
rule among the hydrophobicity components in the order of hydrophilic
substances (HIS) < hydrophobic acids (HOA) < hydrophobic bases
(HOB). The Stokes shift of 1.2 μm–1 is a critical
border, above which the relative fluorescence correlated significantly
with the HOA-related content (Pearson’s r =
0.8). With regard to MW distribution (<1, 1–10, 10–100,
and >100 kDa), SFI was found to be the most sensitive to the change
of MW of <1 kDa proportion, especially at the excitation/emission
wavelengths of 200–320/310–550 nm (r > 0.9). Hydrophobicity-related π conjugation and MW-dependent
light exposure might be responsible for the correlations. These fluorescence
indicators may be useful for convenient monitoring of DOM in wastewater
treatment systems.
While the contact angle is a well-applied indicator of membrane hydrophobicity and surface energy, the interference of surface roughness and porosity in contact angle measurement and surface energy calculation has been long neglected in the field of porous membrane study. We propose an improved method to straightforwardly derive the surface energy of the porous membrane from contact angles with the interference effect corrected. A linearized model was established combining the Young−Dupréand Cassie−Baxter equations, from which the surface energy (Lifshitz−van der Waals and Lewis acid/base components) and roughness index (surface area difference) can be solved simultaneously at a given porosity using contact angles measured with a set of standard polar/nonpolar test liquids. The model solution was examined using hydrophilic microfiltration membranes with different pore morphologies (including perforated plate-like PCTE, irregular particulate bedlike PVDF, and fibrous mesh-like PTFE membranes), with the robustness of the results evaluated via Monte Carlo simulation. In comparison with the verified results of the model solution, it was found that the Lifshitz−van der Waals Lewis acid/base energy values for the tested membranes would deviate by 50−87, 30−160, and 52−97%, respectively, if surface roughness and porosity were neglected in the calculation. The profound effect of roughness and porosity on surface energy determination was further confirmed via theoretical analysis of the Young−Dupréand Cassie−Baxter relationships. This improved approach may apply to the surface energy characterization of hydrophilic rough porous membranes (e.g., hydrophilic microfiltration membranes).
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